The present invention relates to a method of electroplating a sliding bearing with a composite layer of hard particulate incorporated in a metallic matrix, and more particularly, but not exclusively, to bearing shells and thrust washers.

BACKGROUND

Bearing shells for journaled engine bearings typically comprises a strong steel backing layer, a lining layer and an overlay layer that provides the running surface for the journaled shaft, e.g. a hollow generally semi-cylindrical steel backing layer, a copper-based alloy lining layer, and a tin, tin-based alloy or composite overlay layer on the inner surface.

It is desirable to provide increased wear resistance and to improve the fatigue strength of layers in bearing linings, particularly overlay layers. A particular challenge to bearing overlay layer performance is provided by the configuration of vehicle engines to save fuel by using a stop-start operation, in which the engine is stopped each time the vehicle stops, in contrast to conventional engine operation, in which the engine is kept running throughout a vehicle's journey. Engines configured for stop-start operation may restart their engines more than one hundred times more frequently than conventionally configured engines running continuously throughout each vehicle journey. The particular problem that an engine configured for stop- start operation presents arises because engine bearings are conventionally hydrodynamically lubricated, with little or no lubrication initially being provided to the bearings when the engine starts, leading to particularly significant wear during the start-up phase.

It has been proposed to increase the wear resistance of engine bearings by the incorporation of hard inorganic particles, which are substantially insoluble in the electroplating electrolyte, into bearing overlay layers. Exemplary materials are the incorporation of aluminium oxide, silicon nitride, silicon carbide or boron carbide hard particulate into a tin-based alloy matrix. However, the production of such composite layers, with a high concentration of hard particulate, is difficult by conventional electroplating techniques, particularly in a tin-based alloy matrix (e.g. at least 50 %wt tin), and most particularly in the case of a pure tin matrix. SUMMARY OF THE DISCLOSURE

According to a first aspect, there is provided a method of manufacturing a sliding bearing comprising providing a substrate as a cathode in an electrolyte within which a hard particulate is suspended, and depositing a composite layer of hard particulate embedded in a metallic matrix by applying a repeating cycle of bias pulses to the substrate wherein each cycle comprises a high cathodic bias portion and a further bias portion selected from the group consisting of a low cathodic bias portion, a zero cathodic bias portion and an anodic bias portion.

According to a second aspect, there is provided a sliding bearing manufactured according to the method of the first aspect.

According to a third aspect, there is provided an engine comprising a sliding bearing manufactured according to the first aspect.

The method may further comprise agitating the electrolyte to maintain the hard particulate in suspension. The further bias portion may be a low cathodic bias portion.

The high cathodic bias portion may have a bias of at least 125 % of the low cathodic bias portion. The further bias portion may be a zero cathodic bias portion. The further bias portion may be an anodic bias portion.

The absolute value of the anodic bias portion may be between 0.25 and 3.0 times the absolute value of the high cathodic bias portion (i.e. between 0.25 and 3.0 times the magnitude, but of opposite polarity).

The repeating cycle may have a sawtooth profile in which each cycle comprises a monotonically increasing cathodic bias.

The pulse cycle may have a length of 5 to 200 ms, and preferably of 10 to 100 ms. The high cathodic bias portion may consist of 10 to 95% of the pulse cycle.

The high cathodic bias portion may have a peak current density of 0.5 to 10 A/dm ² . The mean average cathodic current density of the cycle is lower than 5 A/dm ² .

The hard particulate may be selected from the group consisting of TiCN, SiC, NbC, Si ₃ N ₄ , Al ₂ 0 ₃ , TiN, and B ₄ C. The suspension may comprise 20 to 200 g hard particulate per litre of electrolyte, and preferably 40 to 100 g per litre.

The metallic matrix may be a pure metal, apart from incidental impurities. The metallic matrix may be pure Sn, apart from incidental impurities.

The metallic matrix may be a metal alloy, apart from incidental impurities. The metallic matrix may be a Sn-based alloy, apart from incidental impurities.

The electrolyte may be a tin methanesulfonic acid electrolyte. The electrolyte may comprise 15 to 80 g/l Sn. The electrolyte may comprise brightener.

The sliding bearing may be a bearing shell or a thrust washer. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

• Figure 1 shows a schematic illustration of a bearing shell;

• Figure 2 shows a first bias pulse profile;

· Figure 3 shows an SEM micrograph of a section of a sliding bearing having an overlay with a B ₄ C hard particulate incorporated into a Sn metallic matrix; and • Figure 4 shows a second bias pulse profile; and

• Figure 5 shows a third bias pulse profile.

DETAILED DESCRIPTION

Figure 1 illustrates a hollow generally semi-cylindrical bearing shell 1 having a steel backing layer 2, a copper-based alloy lining layer 3, a nickel or cobalt diffusion barrier 4, and a composite overlay layer 5 of hard particulate incorporated into a Sn matrix. The bearing shell onto which the composite layer is deposited is provided as a cathode in a bath containing a suspension of hard particulate in an electroplating electrolyte, with an anode formed of a material corresponding to the metallic matrix, e.g. a high purity tin anode.

The electrolyte is a lead-free, tin methanesulfonic acid (MSA) electrolyte (tin ions in methanesulfonic acid), which may comprise performance enhancing additives, such as brightener and anti-foaming agent. For example the electrolyte may be the Bright Tin GBF 30 acidic electrolyte system from Schlotter® Galvanotechnik, which uses a reci pe of Schlotter' s ingredients consisting of 13.0 litres Tin Concentrate FS 20 (which contains 310 g/l tin(ll)), 6.0 litres GBF 31 Starter (20 to 25 %wt 2-naptholpolyglycolether, 1 to 2.5 %wt 1 ,2- dihydroxybenzene, and 1 to 2.5 %wt methacrylic acid), 0.4 litres GBR 32 Brightener (35 to 50 %wt 2-isopropoxyethanol, and 5 to 10 %wt 4-phenylbut-3-en-2-one), 1 1.0 litres GBF 33 Make Up Concentrate (which is 45 %wt MSA), and the balance to 100 litres of deionised water. This forms a solution of 30 to 60 g/l tin, although concentrations of 15 to 80 g/l may be used. The suspension is maintained at a temperature of 20 to 30 °C. The chemical composition and pH is maintained during deposition by replenishment of the consumed chemicals.

Hard particulate, such as boron carbide, alumina, silicon nitride, boron nitride, silicon carbide, niobium carbide, titanium nitride, or titanium carbo-nitride, with a particle size of less than 7 μηι, is suspended in the solution with a concentration of approximately 60 g/l (operation has been demonstrated with 20 to 200 g/l hard particulate, and preferably 40 to 70 g/l). Ultrasonic and/or mechanical stirring agitation is used to maintain the hard particulate in suspension. A cathodic bias (i.e. a negative bias is applied to the cathode relative to the anode) creates a cathodic current (i.e. a negative current, with respect to the anode) that drives the positively charged tin ions towards the sliding bearing cathode, and deposits the tin ions onto the cathode surface. To provide an enhanced incorporation of the B ₄ C hard particulate the cathode bias is cyclically pulsed at with a pulse cycle period of 10 to 20 ms (although operation has been demonstrated with a pulse cycle period of 10 to 40 ms). The peak cathodic current density is between 0.5 and 5.0 A/dm ² , and the mean average current density across the pulse cycle is up to 3.6 A/dm ² .

As illustrated in Figure 2, in one embodiment a bias pulse cycle is used having a high cathodic bias V _H pulse portion ti and a zero cathodic bias V ₀ portion t ₂ . The high cathodic bias portion is applied for up to 95 % of the pulse cycle (preferably between 10 and 95 %), and produces a high cathodic current density.

By using pulsed electroplating, it is possible to uniformly incorporate up to 20 %wt B ₄ C hard particulate into a Sn metallic matrix of a sliding bearing overlay layer. Figure 3 illustrates a sectional view of such a layer, in which the hard particulate 6 appear as dark specks in the metallic matrix of the overlay layer 5.

The rate of metallic matrix deposition under a constant cathodic current is limited by the ionic mobility of the metal ions (e.g. tin ions), due to the presence of a depletion region in the electrolyte, against the cathode surface. Although hard particulate from the suspension adheres onto the surface, slow deposition of the metal ions that occurs under constant cathodic current is inefficient at incorporating the surface particles into the deposited layer, with the particles instead remaining on the surface as the metallic matrix layer grows. In contrast, during the zero cathodic bias portions (and similarly during lower cathodic bias portions or during anodic bias portions), the concentration of metal ions close to the cathode surface is able to increase, leading to a rapid burst of deposition occurring during the high cathodic bias portions, which increases the efficiency of incorporation of the hard particulate into the deposited layer.

Alternatively, as illustrated in Figure 4, the pulse cycle may have an alternating high cathodic bias V _H portions and low cathodic bias V _L portions t ₂ '. The high cathodic bias V _H is at least 1.25 times greater than the low cathodic bias V _L . Additionally there may also be a zero cathodic bias portion (also known as off-time), for example following the high cathodic bias portion. In a yet further embodiment, a double polarity pulse cycle may be used, in which an anodic bias pulse portion (i.e. a reverse bias, relative to the cathodic bias) may be provided. For example, as illustrated in Figure 5, the pulse cycle may have high cathodic bias V _H pulse portion V, an anodic bias V _R pulse portion t ₂ ", a zero cathodic bias V ₀ portion t ₃ ", and a low cathodic bias V _L portion t ₄ ". The anodic bias portion has a bias that is between -0.25 and - 3.0 times the bias of the high cathodic bias portion (i.e. its magnitude is between 0.25 and 3.0 times the magnitude, but of opposite polarity).

Such anodic bias pulses may de-plate metal ions from the deposited layer, providing a high concentration of ions close to the cathode surface, further increasing the subsequent rate of deposition during the high cathodic bias pulse portion, further enhancing the incorporation of hard particulate into the deposited layer of metallic matrix.

The sliding bearing may be a bearing lining or a thrust washer, which is inserted into the bearing assembly of an engine.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of them mean "including but not limited to", and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.